Sleeping consistently for less than six hours a night has been linked to an increased risk of early death. Photograph: Isopress/Rex Features

Sleeping consistently for less than six hours a night may cause an early death, but too much sleep could also mean problems, according to a study that claims to have found unequivocal evidence of the potential harm from abnormal sleep patterns.

The research, by academics in the UK and Italy, analysed data from 16 separate studies across Europe, the US and Asia over 25 years, covering more than 1.3m people and more than 100,000 deaths.

It found that those who generally slept for less than six hours a night were 12% more likely to experience a premature death over a period of 25 years than those who consistently got six to eight hours' sleep. Evidence for the link was unequivocal, the researchers concluded.

It also concluded that those who consistently sleep more than nine hours a night can be more likely to die early. Oversleeping itself is not seen as a risk but as a potential indicator of underlying ailments.

"Whilst short sleep may represent a cause of ill health, long sleep is believed to represent more an indicator of ill health," said Professor Francesco Cappuccio, who led the study and is head of the Sleep, Health and Society programme at the University of Warwick.

"Modern society has seen a gradual reduction in the average amount of sleep people take, and this pattern is more common amongst full-time workers, suggesting that it may be due to societal pressures for longer working hours and more shift-work. On the other hand, the deterioration of our health status is often accompanied by an extension of our sleeping time.

"Consistently sleeping six to eight hours per night may be optimal for health. The duration of sleep should be regarded as an additional behavioural risk factor, or risk marker, influenced by the environment and possibly amenable to change through both education and counselling as well as through measures of public health aimed at favourable modifications of the physical and working environments."

The study noted that previous research into lack of sleep had shown it was associated with ailments including heart disease, high blood pressure, obesity, and diabetes.